This invention relates to an optical system driving apparatus for moving an optical system by a minute distance. The present invention is suitably used, for example, for the driving of the objective lens of an optical head in an optical information recording-reproducing apparatus.
Optical information recording-reproducing apparatuses for applying a laser beam in the form of a spot onto an information recording medium to thereby record information on the recording medium and/or reproduce the information recorded on the information recording medium have been put into practical use.
Such apparatuses include an optical disc apparatus. The optical disc is such that information tracks comprising rows of information pits having a width on the order of 1-2 .mu.m are formed spirally or concentrically. When reproducing information recorded on the rows of information pits, a laser beam is applied in the form of a minute spot to the information tracks of the optical disc from an optical head while the optical disc is being rotated and the rows of information pits are scanned by the beam spot.
When the reflected light or the transmitted light of the light applied to the surface of the optical disc in this manner is detected by a photo-detector, the optical properties of the light entering the photodetector (for example, the intensity, the angle of polarization, etc. of the light) change depending on whether an information pit exists at the beam spot position. By detecting the changes, a reproduction signal corresponding to a row of information pits can be obtained.
Also, in such an optical disc apparatus, it is very important for the minute spot to always scan the rows of information pits on the recording medium accurately. For this reason, auto focusing for correcting an focus deviation resulting from warp or the like of the recording medium and auto tracking for correcting any deviation of application by eccentricity or the like of the recording medium become necessary.
FIG. 1 of the accompanying drawings is a perspective view showing an optical system driving apparatus according to the prior art described in U.S. application Ser. No. 922,855 (filed on Oct. 24, 1986) now abandoned. As shown in FIG. 1, an objective lens holding member 128 is inserted in a support shaft 126 fixed to a base bed 121 as by press-in, through a bearing, and an objective lens 123 and a weight 129 substantially equal in weight to the objective lens 123 are mounted near the support shaft 126. Further, a neutral point holding member, not shown, is mounted below the weight 129. A coil 124 for focusing is wound on the outer periphery of the objective lens holding member 128, and a coil 125b for tracking and a coil 125a for tracking, not shown (which is mounted at the position opposite to the coil 125b for tracking with respect to the support shaft 126) are further mounted on the coil 124 for focusing. Permanent magnets 127a and 127b for focusing having opposed magnetic poles formed near the outer peripheral portion and the inner peripheral portion of the objective lens holding member 128 are secured to the base bed 121, so as to be opposed to the coil 124 for focusing, and permanent magnets 122a and 122b for tracking are secured near the outer peripheral portion of the objective lens holding member 128 so as to be opposed to the coils 125a and 125b for tracking. The objective lens holding member 128 is vertically slid and clockwisely and counterclockwisely rotated by an electromagnetic force produced by an electric current flowing to the coil 124 for focusing and the coils 125a, 125b for tracking and a magnetic field formed for each coil.
An optical system driving apparatus has been constituted by attaching to the optical head body by means of screws, the base bed 121 to which the permanent magnets 122a, 122b for tracking, the permanent magnets 127a, 127b for focusing and the support shaft 126 are secured. The optical system driving apparatus as shown in FIG. 1 wherein the objective lens holding member is clockwisely and counter-clockwisely rotated about the support shaft is disclosed in U.S. Pat. No. 4,467,463, U.S. Pat. No. 4,566,089, U.S. Pat. No. 4,571,026, U.S. Pat. No. 4,643,522,and U.S. Pat. No. 4,687,296, for example.
Also, generally in a DRAW (direct read after write) type or type optical disc apparatus, spiral guide grooves for tracking are provided in advance on the optical disc. The pitch of the guide grooves is very small, say, on the order of 1 .mu.m, and therefore, when a light spot is applied thereto, diffraction occurs and the diffracted light is scattered in a direction perpendicular to the tracks. In a push-pull type tracking apparatus, a variation in light and shade of the patterns of O-order and .+-.1st-order diffracted light beams on a detector for tracking are taken out as a tracking error signal and objective lens servo is effected.
In an optical disc apparatus wherein an objective lens for condensing a minute spot on a predetermined track is moved relative to a detector for tracking to thereby accomplish tracking servo, the presence of great eccentricity of the disc causes the center on the detector to move. Offset to a tracking error signal occurs and thus, it is difficult to accomplish accurate tracking.
This will now be described briefly with reference to FIG. 2 of the accompanying drawings.
FIG. 2 shows a portion of an optical disc apparatus, and more specifically, it shows a portion necessary to effect tracking servo.
In FIG. 2, a light beam 10 from a semiconductor laser (not shown) is condensed on a predetermined track on a disc 8 having tracking guide grooves (not shown) by an objective lens 3. The objective lens 3 follows the track in the tracking direction with the aid of an actuator (not shown) with the eccentricity of the disc. When the eccentricity of the disc 8 is very small, the optic axis of the objective lens 3 is substantially coincident with the center of the light beam 10 from the laser, and a diffracted light beam including the asymmetry of .+-.1st-order diffracted light beamed corresponding to the positional deviation between the track and the spot again enters the objective lens 3. This diffracted light beam passes through the objective lens 3, whereafter it is reflected by a half-mirror 15 and arrives at two-division detectors 14a and 14b. The spots on the detectors 14a and 14b, as shown in FIG. 3(b) of the accompanying drawings, are positioned symmetrically with respect to the dividing line, and a tracking error signal is produced by the asymmetry of the .+-.1st-order diffracted light beams on the detectors 14a and 14b.
If the disc 8 is eccentric by a distance D, the objective lens 3 pursues it and therefore, the light beam from the laser enters the objective lens with the center of the former being deviated from the center of the latter by the distance D. Along therewith, the spots on the detectors enter asymmetrically with respect to the dividing line as shown in FIG. 3(a) or 3(c) of the accompanying drawings and therefore, the differential output of the detectors 14a and 14b causes an offset .delta. as shown in FIG. 4(a) or 4(c) of the accompanying drawings. If servo is effected in accordance with a tracking error signal which has caused the offset .delta., it becomes impossible to confine the beam spot 11 rightly onto the track and it becomes difficult to accurately record or reproduce information.
An example for solving this problem is disclosed in U.S. application Ser. No. 041,348 (filed on Apr. 22, 1987) now Pat. No. 4,853,917. The design is such that in an optical system driving apparatus provided with a light source, an objective lens for condensing the light beam from the light source as a minute spot on a track of a recording medium, and means for moving the objective lens in the tracking direction perpendicular to the track, at least one photoelectric conversion element movable with the objective lens is provided outside of the effective diameter of the objective lens and a part of the light beam from the light source is directly received by the photoelectric conversion element to thereby detect the position of the objective lens in the tracking direction.
However, in the above-described optical system driving apparatus, the objective lens position output is obtained from the marginal portion of the light beam from the light source and therefore, it has been difficult to obtain an output of a desired level. Also, when the distribution of the light beam from the light source is not laterally symmetrical with respect to the optic axis up to the marginal portion of the light beam, it becomes likewise difficult to detect the position of the objective lens in the tracking direction.
In view of the above-noted problems peculiar to the above-described apparatus, the present invention has as its object the provision of an optical system driving apparatus which can accurately detect the position of an optical system such as an objective lens.
The above object is achieved by an optical system driving apparatus provided with a rotatable type optical system holding member having an optical system such as an objective lens system in a portion thereof, a shaft for supporting the movement of the holding member, and a base bed to which the shaft is fixed, characterized in that a constituent of means for detecting the position in the direction of rotation is provided in a portion of the rotatable type optical system holding member.